An Investigation into the Protection of Transmission Grid against Lightning Transient Overvoltages and the Impact of Wind Farm Integration

Abstract

Onshore wind farms have been expanding rapidly globally, and are being installed in areas with high wind density. Often, the aforementioned areas are mainly elevated terrains, which coincide with high soil resistivity and high lightning activity. Moreover, wind farms consist of several individual wind turbines dispersed over large areas. Additionally, most wind farms comprise highly elaborate underground cable network connected to grounding grids and individual power transformers. The entire wind farm power system is integrated into the conventional overhead transmission grid to transmit the generated renewable energy to the main power grid. In regions with high levels of lightning incidences, the integrated power system is exposed to severe lightning strikes, resulted in transient overvoltages impressed on individual wind turbines, or on the transmission grid. The effects of the transient overvoltages are exacerbated in areas with high soil resistivity. These conditions can have adverse impacts on the performances of the integrated power grid, especially on sensitive wind turbine electrical systems. Hence, this thesis presents investigations into lightning interactions with the hybrid power system, which comprises modern wind turbines and a conventional power grid. The research develops test networks for both a conventional transmission grid and a wind farm, which are suitable for transient overvoltage analysis. The impacts of lightning induced transient overvoltages on the hybrid power system are examined using the well-known electromagnetic transient program (EMTP) software package. Thus, the test networks are implemented and simulated in EMTP package. The simulation results are used to provide improvements to existing lightning protection systems. Also, the research presents improvements to the techniques being used to reduce shielding failure flashover and back flashover rates on transmission lines. Furthermore, the research proposes novel methodology to estimate the failure rate of wind turbine electrical components exposed to lightning flashes. The proposed method accounts for both upward and downward lightning strokes to compute the components failure rates. It utilises the leader progression models to calculate the lightning incidences on wind turbine blade. The proposed method is compared with the collection area procedure, suggested by the IEC 61400-24:2010. The results show that the proposed method predicts lower failure rates of wind turbine electrical components than those predicted by the IEC 61400-24:2010 technique.

Date of Award	6 May 2015
Original language	English
Awarding Institution	University of Bath
Supervisor	Raj Aggarwal (Supervisor)